In a background art, there is known a hoist including a drive shaft for driving a load sheave, a pressure receiving member fixed to the drive shaft, a drive member extractably and retractably screwed to the drive shaft, and a rotation drive member rotatably fitted to the drive member for transmitting driving of a drive wheel of a hand wheel or the like to a press drive member. In such a hoist, there is known an overload preventing apparatus for facilitating adjustment of a restricting load of a hanging load and easily carrying out hoist down even when brought into an overload state including a belleville spring for imparting a bias force to the press drive member and the rotation drive member. (for example, refer to Patent Reference 1)
The overload preventing apparatus described in Patent Reference 1 will be explained as follows in reference to FIG. 14 and FIG. 15.
FIG. 14 is a front view showing a hoist of a background art, FIG. 15 is a disassembled perspective view showing an essential portion of an overload preventing apparatus of the hoist. In FIG. 14, a drive shaft 22 is rotatably inserted to a load sheave 21. The drive shaft 22 is formed with a screw portion 22a, the screw portion 22a is screwed with a pressure receiving member 24 and a drive member 30 from a side proximate to the load sheave 21, and the pressure receiving member 24 is fixed to the drive shaft 22 by being screwed to an innermost portion of the screw portion 22a. The pressure receiving member 24 is concentrically provided with a disk portion 24a having a large diameter and a boss portion 24b having a small diameter, and the boss portion 24b is outwardly fitted with a reverse rotation preventing ring 27 by being interposed by a pair of friction members 29, 29. The reverse rotation preventing ring 27 and the friction members 29, 29 arranged on both sides thereof are constituted to be able to be pressed to a disk portion 24a of the pressure receiving member 24 by the drive member 30. The reverse rotation preventing ring 27 includes locking teeth 27a inclined to one side in a circumferential direction at an outer periphery thereof, by engaging the locking teeth 27a with a ratchet claw 28 axially supported by a side plate, the reverse rotation preventing ring 27 is prevented from being rotated reversely and is made to be rotatable in one direction, that is, a hoist up direction relative to the drive shaft. Further, in FIG. 15, locking teeth 30c having the same shape are formed at a circular disk portion constituting a front end face in an axial direction of a flange portion 30a of the drive member 30 and on an outer side of a boss portion 30b having a large diameter. The boss portion 30b having the large diameter of the drive member 30 is outwardly fitted with a rotation drive member 32, and locking teeth 32a engageable with the locking teeth 30c of the drive member 30 are projected to be formed on a base end side in an axial direction at a base end face in an axial direction of the rotation drive member 32. The respective locking teeth 32a of the rotation drive member 32 are formed by a shape substantially adapted to recess grooves formed among the locking teeth 30c of the drive member 30. A hand wheel 34 is outwardly fitted to an outer peripheral portion of the rotation drive member 32. Positioning of the rotation drive member 32 relative to the drive member 30 is carried out by screwing a nut 36 to a screw portion of a boss portion 30d having a small diameter on a side of the front end of the drive member 30 by way of a rotation restricting member 35 in a circular plate shape and a belleville spring 33 constituting urging means. At an inner peripheral portion of the rotation restricting member 35, a plurality of pieces of engaging projected portions 35a substantially in a short shape are projected to be formed on an inner side in a diameter direction. Although the rotation restricting member 35 is restricted from being moved in a peripheral direction relative to the drive member 30, but the rotation restricting member 35 is made to be able to be moved in an axial direction. The belleville spring 33 operates an urge force to press the rotation drive member 32 in a direction of the base end in the axial direction (side of the drive member 30) by way of the rotation restricting member 35.
Next, operation of the hoist of the background art will be explained. First, the nut 36 is screwed, and the belleville spring 33 constituting the urging means presses the rotation restricting member 35 to the side of the base end in the axial direction. The rotation restricting member 35 is brought into contact with the rotation drive member 32, and therefore, the rotation restricting member 35 urges the rotation drive member 32 to the side of the drive member 30. At this occasion, the locking teeth 30c of the drive member 30 and the locking teeth 32a of the rotation drive member 32 are engaged with each other. In a case of hanging down a load equal to or smaller than the restricting load from a load chain wound around the load sheave 21, when the rotation drive member 32 is rotated by operating the hand wheel 34, rotation is transmitted to the drive member 30 by way of the locking teeth 32a, 30c, and the load can be hoisted up by pressing to rotate the pressure receive member 23 by the drive member 30. In contrast thereto, in a case of hanging up an overloaded load, when the rotation drive member 32 is rotated by the hand wheel 34, while pressing back the rotation drive member 32 to the side of the front end in the axial direction against the urge force of the belleville spring 33 along with the hand wheel 34, press faces of the locking teeth 32a in hoist up are pressed up along press faces of the locking teeth 30c of the drive member 30 in hoist up, the locking teeth 32a of the rotation drive member 32 ride over the locking teeth 30c of the drive member 30, and are contained to next grooves among the locking teeth 30c of the drive member 30 to be engaged therewith by the urge force of the belleville spring 33. In this way, when the hand wheel 34 is rotated in the hoist up direction under the overload state, although the drive member 30 is not rotated, only the rotation drive member 32 is rotated, the drive member 30 cannot be rotated regularly, and the overload is prevented from being hoisted up (wound up).    Patent Reference 1: Japanese Patent Specification No. 3096290 (refer to pages 3 through 5, FIGS. 1, 2)